This is a version of PKCS7_get_certificates but does not require
crypto/x509.
BUG=54
Change-Id: I20152a8d1f3ed866d47e41fe576ea9f442490224
Reviewed-on: https://boringssl-review.googlesource.com/15129
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>